As some knowledge of the science of equilibrium and motion of fluids is essential to the intelligent study of afironautics, it seems advisable to present a few of the main principles of that science illustrated by practical applications.
Afiromechanics may be divided into two branches : a6rostatics and a6rodynamics. Afirostatics is the science of equilibrium of a gaseous fluid; a6rodynamics is the science o.f motion and resistance of such a fluid. The two branches may be treated in succession. We shall begin with a6rostatics, or, still better, with the general science of fluid statics.
When a fluid is at rest, o.r moving with equal velocity at every point, it can offer no resistance to change o4 shape; that is, it can sustain no shearing force. Its pressure against any surface is, therefore, normal to that surface. From this it easily follows that the pressure at any lx)int of a static1 fluid is equal in all directions; for otherwise a short filament o.f fluid at the point, extending in any two directions .of unequal pressure, would move in the direction of the lesser pressure, since the filamental wall can exert no tangential restraining force. being assumed frictionless. Hence the term hydrostatic pressure is commonly used for the stress, or force, acting at a point in a material substance, when no tangential or shearing effort is present.
This hydrostatic condition obtains in natural fluids only when they are at rest or moving without change of shape. In perfect or frictionless fluids, which are purely hypothetical, the pressure at every point is hydrostatic, whether the fluid be still or moving without acceleration; but in natural fluids, such as air or water, if there is shearing motion of any kind, there is sure *Abstracted from the writer's forthcoming popular treatise on afiromechanics.
1 By static fluid is meant one that is not changing shape or velocity.
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* Centroid mean~ centre of mass, which is commonly taken as the centre of gravity.
VOL. CLXXlII, No. Io34--II